In joining roof parts such as a roof panel and a roof side rail of a bodywork of an automobile, a spot joining technique has generally been employed. Because two or more parts such as a roof panel and a roof side rail are joined together to form a bodywork, the bodywork usually has joined portions having a U-shape cross section known as Mohican portions by their shape. From the viewpoint of aesthetic of these joined portions, spot weld marks are usually covered with roof molding parts.
Meanwhile, in order to realize what is known as a Mohican-less structure not requiring such roof molding parts, a method has been studied with which a roof part and its peripheral parts are joined continuously rather than by spot joining. Practical application of joining by laser brazing, which involves a relatively low heat input, has also been studied. Moreover, joining by MIG brazing at a low heat input has also been studied.
PTL 1 discloses an example of joining through laser brazing by disclosing an invention related to a laser brazing method with which the decrease in joint strength of a brazing joint due to the laser is suppressed and stabilization of the joint strength is achieved. PTL 2 discloses a technique of joining bodywork parts together by MIG welding, for example.
The braze joining method described above has an advantage in that there is no need to fill gaps with weld sealers after welding unlike in spot welding. However, the braze joining method has a difficulty in putting together a structure with high precision although it involves lower heat strain compared to typical welding. In this respect, there have been proposed a technique (refer to PTL 3) of changing the structure of roof parts, i.e., workpieces, to a structure that undergoes less heat strain in braze joining and a technique (refer to PTL 4) directed to formation of weld beads.